Integration by means of an overlay network involves the use of a separate network and interface hardware where the subsystem manufacturers merely supply a connection into their system.  The physical cable and protocol is designed by the supplier of the integration system.  All the subsystems are connected onto the overlay network at nodes or points on the network.  The subsystems use their own protocols, hardware, and main communications between their controllers and devices and may have their own graphical user interface.

With this solution, a system provider would supply interface nodes or points which connect to each subsystem, using RS232 or RS485 standards for example.  Each point or node then has a driver contained within it which makes it possible to access information on the overlay system by converting data requests and responses between the two protocols.  Therefore, data is transferred across the overlay network and only converted into another protocol by the driver connected to the system.

Using a network overlay system, full integration can take place whilst allowing subsystem manufacturers to supply their systems as they are, without adhering to any specific or agreed standards. The subsystem providers can use their preferred standards and networks to ensure the best solution for each system. For example, a fire system may work best with a Bus technology and a secure protocol, whereas a non-life-essential BMS system may be more effective using a current loop network.

Viewing the system
A user interface is usually provided by the overlay systems manufacturer. This could take the form of a web based front end or a dedicated software package. With the overlay system the end user can still select to have the subsystems provider install their user interface as well. With an overlay system, addressing and displaying system information (points) uses standard and common objects which have been made available by the overlay system provider. Usually the overlay system providers will make as much information as possible available for viewing and adjustment. Unlike the single network, there is no requirement to register to a standards keeper and therefore new items available in a subsystem can be implemented quickly. In the case of North Building Technologies, it is possible for system partners to develop specialist user interfaces by connection to the North drivers using connectivity provided by North for most Windows development systems.

Sharing information
Most overlay systems provide the sharing of information by setting up communications between one node or point and another. This is usually achieved using some simple software provided by the overlay system provider and is very low cost. With an overlay system then, it is possible to share information and control on the overlay network and in the individual subsystems. Essentially, the overlay system provides the integration of the systems whilst allowing subsystem suppliers to develop and supply systems which lend themselves purposely to their application. Therefore engineers will look after and commission in their own discipline and the integration can then form another discipline which also avoids any conflict of interests.

Advantages

• Subsystems created in commercial and intellectual safety which lend themselves well to their particular application

• inexperienced engineers crossing over into other disciplines

• Third party handling of subsystem protocols and development

• No restrictions on development and creativity, and also flexibility for the subsystem suppliers in not having to conform to any particular standard

• An overlay system would be developed to handle integrated traffic, not all-systems traffic control and information sharing is possible on the overlay network and the subsystems networks

• Standalone operation will continue for each system if loss of the overlay network occurs

Disadvantages

• As with subsystems, a single supplier of the integrated overlay solution

• More cable installations with multi-network installations, usually one per system

• No interchangeability of devices either on the same type of system or between different systems

• Updating protocols in subsystems require the update of overlay system drivers

• To enable integration to take place, the protocols need to be supplied to the overlay system provider together with hard ware to enable driver development. This can sometimes be difficult to arrange with certain subsystems providers